1. Field of the Invention
This invention relates to methods and apparatus for the formation of multi-layer laminates of thermoplastic materials in an extrusion system to form diverse thermoplastic materials into multiple layers of desired thicknesses, and more particularly to an infinitely adjustable extrusion die or adaptor for a die for the extrusion of thermoplastics over a wide range of relative thicknesses and viscosities.
2. Background of the Prior Art
This invention is concerned with the art of variable area orifices or nozzles utilized with plastic extruders for the manufacture of plastic sheet or film. It is known in the art that many advantages are achieved by the production of multi-layer film constructions of thermoplastics as these materials enable a combination of properties not presently obtained in a layered structure. Originally such products were prepared principally by laminating separately formed films or sheets together by adhesives, heat or pressure. More recently however, techniques were developed for melt laminating, that is two or more diverse thermoplastic materials from separate molten layers converge and join under pressure within the die to emerge as a single laminated material. Such processes make use of the laminar flow principle which enables the two or more molten layers to join in a common flow channel without intermixing thereof at the contacting interfaces under proper operating conditions.
The present invention therefore is concerned with so-called multiple layer extrusion systems which has come into recent prominent use as they provide a convenient way of melt lamination, co-extrusion coating and the like, but more particularly provide for formation of multiple layers of similar or dissimilar thermoplastic materials. This invention is more particularly concerned with so called multi-manifold die systems. In general, such dies or adaptors are designed with an individual flow channel/manifold for each layer and normally the layers are brought into contact just above the exit slot of the die. Typically, each manifold is provided with an adjustable flow restricter means to permit adjustment of flow uniformity of the individual layers. The manifold may be of constant or variable cross section depending on individual requirements and design preferences. Because the layers are joined only near the final exit slot, materials with somewhat diverse rheological properties can be processed but adjustments of the flow restriction means is necessary to minimize individual layer flow nonuniformity and obtain a uniform flow just prior to joining. Any tendency toward actual motion flow at the junction point will cause nonuniformity in the product.
Many devices are known to the art and disclosed in prior patents for the production of laminated films of this type. Much of the prior art is concerned with the use of so-called metering blocks wherein each orifice is controlled by a separated metering block. The metering blocks are adjusted by tension or draw-in bolts or compression bolts with the thread on the shank of such tension or draw-in bolt or compression bolt being a motion thread. Guidance of the metering block is generally affected by means of a groove. U.S. Pat. Nos. disclosing this type of extrusion apparatus are for example 3,464,087; 3,694,119; 3,813,204; 3,877,857 as well as 3,218,971.
Another group of prior art devices of this type make use of one or more mandrels within the die wherein the mandrel is positioned at the mouth of the die cavity and is movable in order to adjust the width of the die cavity and thereby adjust the thickness of the thermoplastic material being extruded. Patents which disclose apparatus of this type include U.S. Pat. Nos. 2,780,835; 3,453,690 and 3,559,239. A patent showing a similar apparatus in U.S. Pat. No. 3,464,087 which makes use of a wedge which moves toward or away from the entrance to the extrusion channel. Still a further device is disclosed in U.S. Pat. No. 3,390,432 to Becker which provides an eccentric means mounted in the body of the die and extending into a slot in the metering block whereby the position of the metering block in the body can be determined by movement of the eccentric means.
In all of these prior art devices however, there is no provision by which infinite adjustment of two, three or more extrusion passages can have their relative dimensions varied so as to provide multi-layer thermoplastic material laminate wherein each of the layers can be a thickness different from any of the other layers.
One prior patent which is concerned with this type problem is U.S. Pat. No. 3,761,211 to Parkinson which discloses an extrusion apparatus die for producing laminated film which includes a die block having a die passage leading to a pair of die lips through which thermoplastic materials can be extruded. The apparatus comprises an adaptor connected to the die block and having a main channel connected to the die passage, feed channels which converge into the main channel and valving means located at the point of convergence of the feed channels for controlling the relative flow of the thermoplastic materials from the feed channels into the main channels. With this system however, it is not possible to control the rate of introduction and combination of melt extrudates of differing viscosity and flow rate. Accordingly, problems are encountered in this system in failing to obtain a uniform laminate at the convergence point.
The present invention provides a method and extrusion system for the formation of multi-layer laminates of diverse thermoplastic materials to obtain uniform lamination.